Demand for digital subscriber line (DSL) service across existing twisted pair copper wires between a central office and a remote location is increasing. Typically, DSL services operate in accordance with DSL standards recommended by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU). A family of DSL Recommendations from the ITU includes: G.992.1, G.992.2, G.991.1, G.996.1, G.994.1, G.997.1 and G.995.1. Recommendation G.995.1 provides an overview of these standards. Recommendations G.991.1, G.992.1, G.992.2 have developed techniques for transmitting a range of bit rates over the copper wires of the local network including high bit rates at relatively short distances, and lower bit rates at longer distances. In particular, the G.992.1 and G.992.2 recommendations are based on asymmetric digital subscriber line technology that has different data rates in each direction of transmission. The G.992.1 recommendation is referred to as G.dmt and requires a splitter, which filters the voicegrade signals at the remote location. The G.992.2 recommendation is referred to as G.lite and does not require a splitter. Recommendations G.994.1, G.996.1 and G.997.1 support the G.992.1 and G.992.2 recommendations by providing common handshake, management and testing procedures. These standards allow substantial flexibility in implementation.
Reed-Solomon encoding is a method of forward error correction used in DSL communications to detect and correct transmission errors, effectively increasing the signal-to-noise ratio of the communications channel. By encoding information, errors may be reduced without decreasing the data rate. In Reed-Solomon encoding, redundant symbols are added to information symbols to allow errors to be detected and corrected. As the number of redundant symbols increases, a greater level of noise may be tolerated. Among the transmission parameters to be selected in DSL communications are FEC parameters for Reed-Solomon encoding. The FEC parameters determine the amount of information data in an information field, and a number of redundancy symbols that are associated with the information field of an information frame.
To initiate a DSL communication session, Reed-Solomon encoding parameters for forward error correction (FEC) are selected. During initialization, the channel is analyzed and FEC parameters are determined and exchanged. While the ITU recommendations proscribe protocols for initialization, the ITU recommendations do not describe how the ATU equipment at the central office (ATU-C) or the remote user location (ATU-R) should act on data produced by the channel analysis or how the ATUs should select FEC parameters. The recommendations set a bit error rate standard (BER) of 10−7, but provide no guidance on selection of FEC parameters to most effectively achieve that bit error rate. Therefore, a method and apparatus of selecting FEC parameters based on available channel analysis data is needed. The FEC parameters should also yield a target bit error rate.
Sometimes burst errors, which cannot be corrected by FEC methods, occur during a communication session. Therefore, the method and apparatus should also correct burst errors.